1. Technical Field
The invention generally concerns power supplies and lighting ballasts, and more particularly to voltage regulation for zero-voltage-switched (ZVS) inverters.
2. Description of the Prior Art
Electronic lamp ballasts include inverters to provide a high frequency substantially square wave voltage output to a load. The load typically includes a resonant circuit and lamps. A typical inverter circuit includes power switching circuitry involving transistors to convert a DC input to a desired high frequency AC output.
A control circuit, such as disclosed in U.S. Pat. No. 4,952,849 to Fellows et al., for driving the switching circuitry of a voltage-fed inverter, senses current flowing through the load to control the switching circuitry. The switches are turned on so as to minimize damage and losses thereto during transition and is commonly referred to as zero-voltage switching (ZVS). The switching technique minimizes the voltage across the switch as the switch is being turned on.
The switching frequency of the inverter is typically above the resonant frequency of the resonant circuit, that is, to maintain the resonant circuit in an inductive mode. More generally, in the zero-voltage switching scheme, it is critical to maintain the circuit in an inductive mode when transitioning between the power switches, regardless of the load type. Otherwise, large power losses and damage to the components within the circuit can result.
One commonly used method to achieve zero-voltage switching is to maintain the switching frequency higher than the resonant frequency, as described in Steigerwald, "A Comparison of Half-Bridge Resonant Converter Topologies," IEEE Transactions on Power Electronics, April 1988, pp. 174-181, incorporated by reference herein. This frequency-based scheme can be easily implemented using voltage controlled oscillators (VCO). However, a critical requirement of such a control method is that prior knowledge of the resonant frequency is needed to determine the frequency sweeping region. In simple applications, for example, in LC resonant inverter-based power supplies and electronic ballasts with a single lamp, when the maximum gain frequency does not change significantly during the load and line changes, the frequency-based control method operates successfully.
However, in some cases, the characteristics of the resonant circuit can change rapidly with lamp loads, bus voltage, ambient conditions or aging of the components. Such changes can affect the operating mode of the resonant circuit. Simple frequency based control methods may be unable to adapt to the new frequency-sweep region and therefore be unable to maintain zero voltage switching.
The circuit of Nalbant, "A New and Improved Control Technique Greatly Simplifies the Design of ZVS Resonant Inverters and DC/DC Power Supplies," Proceedings of IEEE Applied Power Electronics Conference, March 1995, pp. 694-701, achieves zero voltage switching by increasing the operating frequency above the resonant frequency. Switching occurs by setting thresholds on the current flowing in a loading matching network. When one of these thresholds is crossed, switching occurs in the inverter. This approach has the disadvantage that threshold values must be chosen and set, and complex logic must be implemented. If the current values experience large fluctuations, the threshold may not be crossed and a correct switching cycle may be missed, or the wrong operating frequency may result. Here, the inductor current information is not further used for output regulation.
Accordingly, it is an object of the invention to provide various voltage regulation methods in a zero-voltage switching scheme for a power supply or ballast having a voltage-fed inverter which overcomes the above mentioned disadvantages of the prior art.